The invention relates to an internal-combustion engine having an intake pipe, an exhaust pipe and an exhaust gas turbocharger whose compressor is situated in the intake pipe and whose turbine, acting in the engine braking operation as a throttle raising the exhaust gas counterpressure, is situated in the exhaust pipe which, in its section extending toward the turbine, is provided with a valve which can be acted upon by actuators, by way of which valve, as a function of operating parameters of the internal-combustion engine/exhaust gas turbocharger system, a bypass is to be opened which bypasses the turbine.
In the case of internal-combustion engines, which, as supercharged internal-combustion engines, are equipped with an exhaust gas turbocharger, it is generally customary to arrange a blow-off valve, normally also called "waste gate", in the exhaust pipe. In known solutions, this blow-off valve is used as a function of operating parameters for the charge pressure control, for example, the taken-in air quantity, the rotational speed, the charge pressure and/or the throttle valve position of the internal-combustion engine being used as parameters.
From the German Document DE-Z, Linnhoff/Peters/Schindler, "Computer Simulation of the Injector Flow at the Turbine Bypass of a Supercharged Diesel Engine", MTZ Motortechnische Zeitschrift 1987/10, Pages 395-402, it is known that, as the result of the opening-up of the "waste gate", the pressure from the turbine can be lowered in order to thus lower the exhaust work and therefore the full-load consumption of the engine.
The goal of such charge pressure control known in the practice is, for example, the improving of the response behavior of internal-combustion engines supercharged by way of exhaust gas turbochargers for avoiding the so-called turbohole, or for limiting the power of the engine in the upward direction.
Since the turbine of the exhaust gas turbocharger is situated in the exhaust pipe, it virtually forms a throttle and acts as a ram element in the exhaust gas flow. By way of this arrangement, the buildup of an exhaust gas counterpressure is caused and/or promoted in a desirable manner in the engine braking operation, in which case the maximally permissible exhaust gas counterpressure determines the maximal engine braking performance and is defined by the respective loading capacity of the engine.
German Patent Document DE 42 32 400 C1 suggests the use of the retaining effect of the turbine of the exhaust gas turbocharger in the engine braking operation, in which case, by means of an adjustable flow guiding device of the turbine, the throttle cross-section of the turbine can be varied. As the result, the braking performance is to be adjustable, and a separate engine braking device is to be eliminated.
It is an object of the invention to limit this exhaust gas counterpressure to an extent which is still acceptable for the braking operation, in order to avoid damages to the engine.
Within the scope of the invention, this is achieved by providing the use of a valve which limits the exhaust gas counterpressure in the engine braking operation and opens up an access to a bypass surrounding the turbine.
As a further development of the invention, the valve is spring-loaded in the closing direction and is expediently acted upon by actuators which comprise an actuating piston which, in the engine braking operation, can be acted upon by exhaust gas counterpressure in the opposite direction of the spring.
In this case, it is also expedient, for assisting the spring, to act upon the actuators by charge pressure, with the same effect as that of the spring, so that also relatively high exhaust gas counterpressures can be controlled at a spring force which is not excessive and, as the result of the differential pressure relationship between the charge pressure and the exhaust gas counterpressure, changes in the control action, which in this respect occur as a function of the rotational speed, can be utilized.
It was found to be particularly expedient to use the same valve as the blow-off valve for the engine braking operation, on the one hand, and for the combustion operation, thus the working operation of the engine, on the other hand; in the former case, for limiting the exhaust gas counterpressure and, in the latter case, for limiting the charge pressure, for which it was found to be expedient to use the charge pressure for the combustion operation as control pressure counteracting the spring force. In view of the adaptation to the defined spring force, on the one hand, and the available charge pressure, on the other hand. It may be expedient in this case to modulate the charge pressure--in coordination with the actuating force of the spring--, particularly by means of a differential pressure cylinder.
The throttling effect of the turbine in the engine braking operation can be influenced within the scope of the invention by the use of exhaust gas turbochargers in the case of which the turbine-side flow-through cross-section is variable and can be at least largely blocked.
This is conceivable in the case of turbines, in which the flow duct surrounding the turbine wheel has a ring-nozzle-shaped mouth area leading to the flow inlet cross-section of the turbine wheel and, in this mouth area, has at least one set of variable guide baffles with guide blades, by way of which the flow cross-section of the ring-nozzle-shaped mouth area can be adjusted. The use of the invention in conjunction with such exhaust gas turbochargers, whose flow cross-section is variable and can at least largely be blocked, may be expedient for reasons of safety or for improving the overall control action; specifically because, in the case of internal-combustion engines with such exhaust gas turbochargers, the buildup of the exhaust gas counterpressure, as a function of the rotational speed, can, even below the nominal rotational speed of the internal-combustion engine, reach a magnitude which would result in the destruction of the internal-combustion engine.
In particular, the use of the invention was also found to be expedient in the case of exhaust gas turbochargers, in the case of which the flow-through cross-section in the mouth area to the turbine can be varied by the axial displacement of a flow guiding device and can at least largely be blocked off, in which case, along its axial course, the flow guiding apparatus may have, in addition to guiding elements, such as blades, ducts or the like, in an axially offset manner thereto, also more or less closed circumferential sections which, by a covering to the mouth area, at least largely block off the flow-through cross-section. Specifically in connection with such further developments of exhaust gas turbochargers, the use of the invention may be expedient because even slight changes of the clear flow-through cross-section have large effects on the respective exhaust gas counterpressure and, since a corresponding precision adjustment of the guiding device is difficult to control, causes high expenditures, particularly under the extreme conditions existing in an exhaust gas turbine. The invention can naturally also be used in the case of modified solutions with an axially displaceable guiding device, for example, in solutions, in which an additional, coaxially situated shut-off sleeve is used as a shut-off element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.